1. Field of the Invention
The present invention relates to an energy trap type piezo-resonator utilizing a shear mode, and more particularly, it relates to a piezo-resonator having a structure for effectively trapping vibrational energy in a resonance part.
2. Description of the Background Art
FIG. 1 is a perspective view showing a conventional energy trap type piezo-resonator 1 utilizing a shear mode. This piezo-resonator 1 has a rectangular piezoelectric ceramic plate 2, which is polarized from an end surface 2a toward another end surface 2b along arrow P. Resonance electrodes 3 and 4 are formed on upper and lower surfaces 2c and 2d of the piezoelectric ceramic plate 2, respectively.
The resonance electrodes 3 and 4 extend from the end surfaces 2b and 2a toward a central portion, respectively. The resonance electrodes 3 and 4 are overlapped to be opposed to each other through the piezoelectric ceramic plate 2 in its central region.
When an alternating voltage is applied across the resonance electrodes 3 and 4, the region of the piezo-resonator 1 overlapping the resonance electrodes 3 and 4 with each other, i.e., a resonance part, is excited in a shear mode. In this case, vibrational energy is trapped in the resonance part overlapping the resonance electrodes 3 and 4 with each other so that the vibrational energy hardly leaks toward the end surfaces 2a and 2b.
The piezo-resonator 1 is an energy trap type piezo-resonator utilizing a shear mode. Thus, it is possible to fix the piezo-resonator 1 to a case member or a circuit board by mechanically connecting the piezo-resonator 1 in the vicinity of the end surfaces 2a and 2b.
In such an energy trap type piezo-resonator 1, the vibrational energy must be efficiently trapped in the resonance part; otherwise no desired resonance characteristics can be attained since the vibration is inhibited when the piezo-resonator 1 is mechanically held in the vicinity of the end surfaces 2a and 2b.
In the piezo-resonator 1, it is necessary to increase the element length L in order to efficiently trap the vibrational energy. On the other hand, the resonance frequency of the piezo-resonator 1 depends on the thickness of the resonance part, i.e., the element thickness t. The piezo-resonator 1 has a thickness t of about 0.3 mm when its resonance frequency is 4 MHz, while the piezo-resonator 1 has a thickness t of 0.6 mm when its resonance frequency is 2 MHz, for example.
In order to reliably trap the vibrational energy in the resonance part, however, the element length L must be increased in proportion to the thickness t. For example, the piezo-resonator having a resonance frequency of 4 MHz and a thickness t of 0.3 mm must have a length L of about 5 mm while that having a resonance frequency of 2 MHz and a thickness t of 0.6 mm must have a length L of about 10 mm, in order to reliably trap the vibrational energy in the resonance part.
Consequently, the element length L is inevitably increased in the energy trap type piezo-resonator 1 utilizing a shear mode.